Page:A Treatise on Geology, volume 2.djvu/20

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6
CHAP. VI.
A TREATISE ON GEOLOGY.

deed, is that between the cessation of the diluvial floods (whatever they were) and the commencement of the actual stream. Judging from a survey of examples in the North of England, we have no doubt that many of these old river terraces are the remains of estuary deltas accumulated when the sea had wider dominion; and we are strongly impressed with the conviction that it is possible now to point out in certain sheltered spots the pebbly shores which, like the modern Spurn, formed the seaward barrier of these estuaries.

Rock Terraces in Valleys.—There is a peculiar class of terraces in valleys, which indicate in the same manner the successive lowering of the level of descending water (or the successive rising of the land); these terraces are formed by solid rock, with little or no trace of gravel, or other detritus. Such cases are frequent in the mining dales of the North of England, which cut deep into the "Yoredale Rocks" or upper mountain limestone series.[1]

In this varied series of limestone, sandstone, and shale, almost every limestone which overlies shale projects into a terrace; and this sometimes happens to strong sandstones similarly circumstanced. It is easy to see that, as this occurs in many of the branching lesser dales, as well as in the principal valley, it may plausibly be argued that the whole effect is due to atmospheric action. It is probable, however, that this is not a sufficient cause; since additional débris might thus be expected to be falling every day, or, at least, more of this accumulation should remain than we see. We must further observe, that the presumed levels of the water are only clearly marked by continuous terraces when the strata dip nearly in the plane of the valley. It appears, that just as, at this day, a mountain stream crossing the Yoredale Rocks forms waterfalls and cliffs at every ledge of limestone, by the wearing away of the subjacent shales—so the great currents which anciently flowed in the valley

  1. Geol. of Yorkshire, vol. ii.